This invention relates generally to systems for testing integrated circuits, and more particularly to the interface between an integrated circuit wafer or chip and a load board.
Conventionally, an integrated circuit test system is connected to the device under test (xe2x80x9cDUTxe2x80x9d) through a load board, which is connected by coaxial cables to a probe card, which is connected to the DUT itself. The probe card is conventionally a printed circuit board with connections for the coaxial cables, fine needles for making contact with the connection points on the DUT, and printed traces on the printed circuit board connecting each cable to a needle.
The quality of test measurements is affected by the nature of the electrical connections, or interface, between the DUT and the test system. It has long been known that long coaxial cables impair the testing of signals, but there has been little success in correcting the problem, despite numerous efforts directed to solving the problem. The bandwidth of coaxial cable is limited. This bandwidth limitation is caused by a high-frequency physical effect called the xe2x80x9cinternal impedancexe2x80x9d of the cable. This effect can cause signal distortion in the cable. Cable-induced signal distortions include mismatch errors between the probe card and cable and between the load board and cable. Cable-induced effects also include power loss and impedance mismatch errors within the cables, dispersion of the transmitted signal, crosstalk, ground loops, and phase distortion.
Nevertheless, flexible and long cables are desired to facilitate movement of the probe card, and to separate spatially the probe card from the load board and the typically large test system. Because the number of channels of an integrated circuit to be tested can number in the hundreds, the bundle of cables can become unwieldy, reducing flexibility and access to the DUT.
In general, in one aspect, the present invention provides a link based on optical fibers to carry a signal between a test system and a connection point proximate to an integrated circuit device to be tested. The link includes an optical fiber for transmitting the signal, a light source electrically coupled to the link input and optically coupled to the fiber input, and a photodetector optically coupled to the output of the fiber to receive light and electrically coupled to the link output. In other aspects, the invention includes a receiver stage to receive the output of the photodetector, an equalizer stage to receive the output of the receiver stage, a high-pass filter stage to receive the output of the equalizer stage, and a power output stage to receive the output of the equalizer stage or, alternatively, of the high-pass filter stage.
In general, in another aspect, the receiver stage includes a microwave differential amplifier to receive the output of the photodetctor, and the equalizer stage includes a differential amplifier in differential mode with a negative feedback function, that receives the output of the receiver stage and provides a first-order high-pass closed-loop response.
In general, in another aspect, the light source includes a light emitter and a driver stage, a load adapter to receive the signal and to transmit the signal to the driver stage. In another aspect, the invention includes a programmable load configured to receive the signal and to transmit the signal to a load adapter or driver stage. In another aspect, the light emitter is a light emitting diode/or a laser diode and the driver stage produces a DC bias current of about 60 mA coupled to a modulation current derived from the signal of about 30 mA. In another aspect, the driver stage includes an emitter-follower driver. In another aspect, the driver stage includes an FET (field-effect transistor) source-follower driver.
In general, in another aspect, the photodetector is a photodiode biased in photoconductive mode, and the optical fiber is a multimode 100/140 micron tapered-index fiber of less than about five meters in length.
In general, in another aspect, the invention provides bi-directional interface for transmitting signals between a circuit tester and a connection point proximate to a circuit to be tested and includes a first optical fiber link to provide for transmission in one direction and a second optical fiber link to provide for transmission in the other direction, a first directional gate for coupling both the input end of the first link and the output end of the second link to one endpoint of the interface, and a second directional gate for coupling both the input end of the second link and the output end of the first link to the other endpoint of the interface.
In general, in another aspect, the first directional gate includes a normally-closed switch between the input of the first link and the one endpoint of the interface, a normally-open switch between the output of the second link and the one endpoint, a sense switch coupled to the second link and connected, on sensing a signal from the second link, to cause the normally-open switch to close and the normally-closed switch to open, and a delay line between the output of the second link and the normally-open switch, in parallel with the sense switch, and connecting the output of the second link to the one endpoint when the normally-open switch is closed.
Among the advantages of the invention are the following. The invention provides an electrical connection which overcomes the problems of resistive power loss and cross-talk in coaxial cables by substituting a fiber-optic interface system, which provides a bandwidth extending to at least 400 MHz with impedance matching at both ends of this interface, even where the impedances are different at the two ends of the interface. Moreover, the invention eliminates mismatch errors in the interface, reduces the diameter of the cables and cable bundle by a factor of about 2 or 3, provides increased flexibility of cable, and provides cables of any length less than about 100 meters without significant signal dispersion. The invention also provides optical isolation of the signals from each other, from ground, and between the probe card and load board. In particular, source and load are isolated (buffered) from each other, so that mismatch errors between them are reduced, and excellent impedance match to source and load is achieved.
The use of long, light and flexible fiber-optic cables makes it possible to extend the probe card away from the load board, to manipulate the position and angle of the probe card with moderate force, and to make the probe card more accessible.
Other advantages and features will become apparent from the following description and from the claims.